The threshold voltage of a MOS transistor is affected by four factors: (1) the concentration of impurities in the silicon substrate, (2) a surface potential energy of the interface between the silicon substrate and an oxide film on the substrate, (3) an electrostatic capacitance of the silicon oxide film and (4) a flatband voltage.
The flatband voltage is further influenced by four factors: (5) the difference in a work function between the silicon substrate and a top electrode, (6) charge distribution in the oxide film, (7) a fixed charge quantity in the vicinity of the interface between the silicon substrate and the oxide film and (8) the electrostatic capacitance of the silicon oxide film.
Generally, the threshold voltage of a semiconductor element, such as a MOS transistor, takes a single particular value which is determined as its characteristic in a certain type of the transistor, whereas a plurality of selected values can be set in a transistor of another type by controlling its drive conditions including the gate, source and drain electrode voltages. In the latter case, the threshold voltage of the transistor in this specification indicates a threshold voltage set under a specific drive condition.
In a conventional technique used for controlling a threshold voltage, it is widely known to introduce an impurity, such as boron or phosphorus, intentionally into a silicon substrate in a transistor manufacturing process to thereby control the concentration of the impurity in the silicon substrate or the surface potential energy on the interface between the silicon substrate and its oxide film. It has been already proposed in one of special-purpose MOS transistors to use a top electrode of a different material thereby to control the difference in work function between the silicon substrate and the top electrode. Further, it is known to introduce an impurity, such as calcium, cesium or iodine, into the interface between the silicon substrate and the oxide film by ion implantation, so that the fixed electric charge quantity in the vicinity of the interface between the silicon substrate and its oxide film is controlled, thereby controlling the threshold voltage of the transistor.
The above matters are disclosed, for example, in S. Aronowitz et al., "Modification of interfacial charge between SiO.sub.2 and silicon", Appl. Phys. Lett., 52(11), pp.913-915 (1988); L. Krunsin-Elbaum et. al, "Shifts on the flatband voltage of metal-oxide-silicon structure due to iodine in SiO.sub.2 ", Appl. Phys. Lett., 48(2). pp. 177-179 (1986); and L. Krunsin-Elbaum, "Dependence of Flatband Voltage of Si-MOS on Distribution of Cesium in SiO.sub.2 ", J.Electrochem. Soc., Vol. 188, No. 8, pp.1712-1715 (1986).
It is expected, in such conventional techniques, that as the integrated circuits become smaller in size, the number of manufacturing steps or the process time involved in a technique for control of the threshold voltages will increase. In addition, it is also expected that various adverse effects will occur in the process for introducing an impurity. The accompanied process would decrease the performance of the MOS transistor with regard to the dielectric breakdown voltage of the silicon oxide film and the lifetime of minority carriers in the silicon substrate, resulting in reduction of the yield of the integrated circuits.
In contrast, if the threshold voltage is controlled through the distribution of electric charges in the oxide film, the above-mentioned adverse effects will be reduced. This is understood by examining each of the four respective factors (5)-(8) contributing to the flatband voltage. However, a practical method for that purpose has not yet been proposed.
It is known that a change in the threshold voltage is caused by a change in the flatband voltage due to a metal element present in a MOS transistor. In most of the metal elements, however, the change of the flatband voltage is almost caused through a fixed charge quantity in the vicinity of the interface between the silicon substrate and an oxide film on the substrate. No metal element has been discovered which is capable of controlling the threshold voltage of the transistor through distribution of electric charges in the oxide film.
It is considered that the presence of a metal element in a MOS transistor would cause a problem such as an adverse effect on electrical characteristics other than the flatband voltage, for example, the dielectric breakdown voltage of the silicon oxide film and/or the lifetime of minority carriers in the silicon substrate. In order to avoid such a problem, implantation of a metal element into the silicon oxide film is generally avoided intentionally. It is known that chromium in the MOS transistor would change the flatband voltage, but would also simultaneously cause a reduction of the lifetime of the minority carriers and an adverse effect on the dielectric breakdown voltage of the silicon oxide film.
The above matters are disclosed, for example, in M. Takiyama et. al, "Electrical characteristics of MOS diode contaminated with Cr-1", 38th combined Applied Physics and related field society lecture preprint No. 2. 30a-SY-14, p. 707, 1991; "Thermal Process Dependence of Chromium Donor/Acceptor in Silicon" in Material Science Forum, Vol. 115-118, edited by T. Taguchi, Trans. Tech. Pub., 1993, pp.261-266; and T. Nakanishi et. al, "Reliability of Metal-implemented SiO.sub.2 ", 38th combined Applied Physics and related field society lecture preprint No. 2. 28a-V-7, p.592, 1991.
We have discovered that the presence of chromium only in the interface between the silicon oxide film and the top electrode in the silicon oxide film in the vicinity of the interface eliminates the above problems to thereby control the threshold voltage.